In recent years, the drilling depth of oil wells and gas wells has tended to increase more and more. It is strongly required to increase the strength of oil well pipe in order to increase the collapse strength of casings and the like. Recently, in order to reduce the drilling costs, there has been increasing demand for an electric resistance welded steel pipe which is high in strength and which does not undergo heat treatment on the entire steel pipe after pipemaking.
In order to increase the strength of the steel material, it is effective to include a sufficient amount of carbon. An electric resistance welded steel pipe having a carbon content of 0.25 mass % or more and having a tensile strength of 800 MPa or more as formed has been proposed (see PLT 1 (Japanese Patent Publication No. 07-102321A)). However, if including a large amount of carbon to increase the strength, the toughness of the steel material may decrease.
Further, in order to increase the strength of the steel material, it is also effective to use a martensite or bainite or other hard metal structure comprising a low temperature transformation structure. Further, in the process of producing the electric resistance welded steel pipe, the pipe is shaped and sized etc., so work hardening by cold working can be used for improving strength. An electric resistance welded steel pipe having a tensile strength of 862 MPa or more as formed by combining such structure strengthening and work hardening has been proposed (see PLT 2 (International Publication No. 2012/144248)).